QA/mininode: Send all headers upfront in send_blocks_and_test to avoid sending an unconnected one

Summary:
PR description:

While this doesn't currently trigger any problems, the network protocol does expect headers to be sent connectable in normal circumstances, and if too many are sent out of order will disconnect the peer.

Backport of Core PR15897

Test Plan: ninja && ninja check-functional

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7778